1. Field of the Invention
This invention relates to a nonaqueous electrolyte cell having a high electric capacity, a high voltage and improved charge-discharge cycle performance.
2. Prior Art
For nonaqueous electrolyte cells having alkali light metals such as lithium and sodium as a negative electrode active material, it is well known in the battery art to use metal oxides, halides and sulfides as a positive electrode active material. Most of commercially available primary cells currently use manganese dioxide or carbon fluoride as a positive electrode active material.
A number of proposals have also been made for rechargeable secondary cells. For lithium secondary cells, for example, compounds having improved cycle performance of occluding and releasing lithium ion are used as the positive electrode active material, for example, metal compounds such as titanium sulfide, molybdenum sulfide, and vanadium oxide and electroconductive polymeric materials such as polyaniline. It is also proposed to use as the negative electrode active material alloys of lithium and aluminum or another metal capable of forming with lithium an alloy, typically a fusible alloy, principally for the purpose of avoiding a shortcircuit problem caused by dendrite growth from metallic lithium used alone. Some secondary cells are now commercially available which use a particular combination of positive and negative electrodes as mentioned above.
Although a variety of materials are available as the positive electrode active material, manganese dioxide among others is expected to be applicable as the positive electrode active material to secondary cells s well as nonaqueous electrolyte primary cells because of its advantages of economy, chemical stability and high voltage.
The manganese dioxide, however, has many drawbacks at the same time. For example, primary cells based on manganese dioxide show poor flatness of discharge voltage irrespective of a somewhat higher discharge voltage as compared with carbon fluoride. When the cell is used in an equipment to be operated at a relatively high voltage, the cell fails to afford a necessary voltage at a later stage of discharge, resulting in a low energy capacity.
The manganese dioxide has the following problems when applied to secondary cells in which charge-discharge operation is repeated. In the case of lithium secondary cells, for example, manganese dioxide can occlude lithium ions between layers thereof upon discharge to convert into a material represented by Li.sub.x MnO.sub.2 where 0.ltoreq.x .ltoreq.1 so that it can take in at most 1 mol of lithium ion per mol of MnO.sub.2. Charging induces reaction of releasing lithium atoms. However, the charging reaction does not proceed to a full extent that the lithium atoms occluded be completely released upon charging, leaving 0.3 to 0.5 mol of lithium in the manganese dioxide. There are present 0.3 to 0.5 mol of dead lithium atoms. The dead lithium atoms mean that they are in an electrochemically unchargeable state. In charge-discharge cycles, the cell shows a substantially reduced capacity retentivity in the second cycle as compared with the initial capacity. Since the amount of inactive lithium gradually increases with the subsequent cycles, the so-called coulomb efficiency lowers and the capacity continues reducing. These problems throw doubt on the adequacy of manganese dioxide to secondary cells.
Many attempts have been made to solve the problems as by controlling conditions of a synthetic process and a subsequent heat treatment so as to optimize the crystalline structure and grain size of manganese dioxide. These methods have not been fully successful.